Hydrants are aboveground extensions of underground water mains, equipped with valves and/or connections for attaching fire hose. The pressure containing parts are usually made of cast iron or ductile iron. The valve seat, valve guide and other important working parts are usually made of bronze. A weather cap on the operating stem nut prevents the accumulation of ice, which would cause the hydrant to bind. Hose nipples are threaded, leaded and pinned, or twist-locked and set-screwed into the hydrant upper body. The valve is opened or closed by a 15 in. (380 mm) long wrench fitted onto the operating stem nut. To prevent freezing, any water in the hydrant barrel drains out at the bottom through a small valve which opens simultaneously as the hydrant is shut down after use. The counterclockwise direction to open is required. Hydrants with pumper connections are not recommended for installation within plant yards. Hydrants can usually be obtained with standardized mechanical joint inlet or flanged inlet connections. Other types of inlet connection are mentioned in the specific manufacturer's listings. Standard outlets are for 2 1⁄2 in. (64 mm ) hose size. Hydrants can be obtained with independent hose gate valves. Hydrant sizes shown are the inside diameter of the main hydrant valve. Unless otherwise noted in the listing, the hydrants have 175 psi (1205 kPa) rated working pressure. Some hydrants, referred to as traffic types, have intentional sections of weakness near the ground line which fracture readily when struck with sufficient force by a moving motor vehicle. They are used to minimize damage to the main hydrant valve and simplify repairs. Unless otherwise specified, the standard FM Approved hydrant inlet connection is 6 in. NPS.